Achromatic fiber optic coupler

ABSTRACT

An achromatic fiber optic coupler of the type wherein first and second single-mode optical fibers, each having a core and a cladding, are fused together along a portion of the lengths thereof to form a coupling region. A matrix glass of lower index than the fiber claddings surrounds the coupling region. The fiber diameters are smaller in the coupling region than in the remainder of the fibers. The refractive index n 2  of the cladding of the first fiber is different from the refractive index n 2  &#39; of the cladding of the second fiber, the difference between the refractive indices n 2  and n 2  &#39; being such that the coupler exhibits very little change in coupling ratio with wavelength over a band of wavelengths.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No. 447,796(G.E. Berkey 20) entitled "Chlorine-Doped Optical Component" filed oneven date herewith.

BACKGROUND OF THE INVENTION

This invention relates to single-mode fiber optic couplers that arecapable of effecting a relatively uniform coupling of light from onefiber to another over a relatively broad band of wavelengths.

Coupling occurs between two closely spaced cores in a multiple coredevice. Fiber optic couplers referred to herein as "fused fibercouplers" have been formed by positioning a plurality of fibers in aside-by-side relationship along a suitable length thereof and fusing thecladdings together to secure the fibers and reduce the spacings betweenthe cores. The coupling efficiency increases with decreasing coreseparation and, in the case of single-mode cores, with decreasing corediameter.

European published patent application No. 0302745 teaches that variouscoupler properties can be improved by inserting the fibers into acapillary tube prior to heating and stretching the fibers, therebyresulting in the formation of an "overclad coupler". After the fibershave been inserted into the tube, the tube midregion is heated to causeit to collapse onto the fibers; the central portion of the midregion isthereafter drawn down to that diameter which is necessary to obtain thedesired coupling. The coupling region of an overclad coupler ishermetically sealed, and the optical characteristics thereof arerelatively insensitive to changes in temperature. The tube also greatlyenhances the mechanical strength of the coupler.

Identical optical fibers are used to make overclad couplers referred toherein as "standard couplers", the coupling ratio of which is verywavelength dependent. A standard coupler which exhibits 3 dB coupling at1310 nm cannot function as a 3 dB coupler at 1550 nm because of thatwavelength dependence. A 3 dB coupler is one that couples 50% of thepower from a first fiber to a second fiber. A standard coupler can becharacterized in terms of its power transfer characteristics in a windowcentered about 1310 nm, which is referred to as the first window. Forexample, a standard coupler might exhibit a coupling ratio that does notvary more than about ±5% within a 60 nm window.

It has been known that an achromatic coupler, the coupling ratio ofwhich is less sensitive to wavelength than it is for a standard coupler,can be formed by employing fibers having different propagationconstants, i.e. by using fibers of different diameter and/or fibers ofdifferent refractive index profile or by tapering one of two identicalfibers more than the other. There is no widely accepted definition ofachromatic couplers. The least stringent definition would merely requirean achromatic coupler to exhibit better power transfer characteristicsthan the standard coupler in the first window. More realistically, thespecification is tightened by requiring an achromatic coupler to performmuch better than the standard coupler in that first window, or torequire it to exhibit low power transfer slopes in two windows ofspecified widths. These windows might be specified, for example, asbeing 100 nm wide and centered around about 1310 nm and 1530 nm. Thesewindows need not have the same width; their widths could be 80 nm and 60nm, for example. An optimally performing achromatic coupler would becapable of exhibiting low values of coupled power slope over essentiallythe entire single-mode operating region. For silica-based optical fibersthis operating region might be specified as being between 1260 nm to1580 nm, for example. It is noted that the total permissible variationin power includes insertion loss and that the permissible powervariation specification becomes tighter as insertion loss increases.Furthermore, for a 3 dB coupler, for example, the coupled power at thecenter of the window should be 50%. If the 50% coupling wavelength isnot at the center of the window, the coupled power specification becomeseven tighter.

In the following discussion, the relative refractive index differenceΔ_(a-b) between two materials with refractive indices n_(a) and n_(b) isdefined as

    Δ.sub.a-b =(n.sub.a.sup.2 -n.sub.b.sup.2)/2n.sub.a.sup.2 ( 1)

For simplicity of expression, Δ is often expressed in per cent, i.e. onehundred times Δ.

A usual requirement for fiber optic couplers is that the fibersextending therefrom, referred to herein as "pigtails", be optically andmechanically compatible with standard system fibers to which they willbe connected in order to minimize connection loss. For example, theoutside diameter and the mode field diameter of the coupler pigtailsshould be substantially the same as those of a standard fiber. One ofthe fibers employed in the fabrication of the coupler can be a standard,commercially available fiber. That feature of the other fiber that ismodified to change the propagation constant should affect the outsidediameter and mode field diameter of the pigtail portion of the otherfiber as little as possible.

U.S. Pat. No. 4,798,436 (Mortimore) discloses a 3 dB fused fiber couplerwherein different propagation constants are obtained by pretapering oneof the fibers. First and second identical standard fibers can be used toform such a coupler. The central portion of the first fiber is initiallyheated and stretched such that the core and the cladding diameterthereof in the tapered region is smaller than the core and claddingdiameter of the second fiber. The pigtail portions of the stretchedfiber can be connected with low loss to a standard system fiber sincethe ends thereof are identical to the ends of the stretched fiber.However, since a separate prestretching operation is employed for eachcoupler made, and since fiber diameter varies continuously along thelength thereof, it is difficult to maintain process reproducibility.Also, a pretapered fiber is fragile and difficult to handle.

U.S. Pat. No. 4,822,126 (Sweeney et al.) teaches a 3 dB fused fibercoupler wherein Δ_(cores), the relative refractive index differencebetween the two coupler cores, is 0.061%. The value of Δ_(cores) isobtained by substituting the two core refractive indices of the Sweeneyet al. patent into equation (1) and solving for Δ. It is apparent fromFIG. 6 of the Sweeny et al. patent that the value of Δ_(cores) shouldhave been greater than 0.061% in order to have achieved goodachromaticity with standard diameter fibers. However, when Δβ isobtained by employing fibers having such large differences between thecore refractive indices, the mode field diameter of one of the couplerpigtails differs sufficiently from that of a standard fiber that it willnot efficiently couple to the fibers of the system in which the coupleris utilized. Rather than increasing the difference between the corerefractive indices to provide a Δ_(cores) greater than 0.061%, Sweeneyet al. maintained that value of Δ_(cores) and, in addition, etched thefiber claddings in order to improve achromaticity.

The Sweeny et al. patent states that although wavelength independence isachieved, as contemplated therein, by having the cores of differentindices of refraction, similar results could be achieved by keeping thecores at like indices of refraction and making the claddings onedifferent from the other with respect to indices of refraction. It willbe obvious from the following discussion that it is impossible to formachromatic overclad-type 3 dB couplers wherein the difference betweenthe refractive indices of the fiber claddings is such that Δ_(clads) is0.06%, assuming that the core and cladding diameters of the two fibersare identical. The value of Δ_(clads) is obtained by substituting thecladding index n₂ ' of one fiber and the cladding index n₂ of the otherfiber for n_(a) and n_(b), respectively, of equation (1) and solving forΔ.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a single-mode achromaticfiber optic coupler that is characterized by a very small change incoupled power over a wide band of wavelengths. Another object is toprovide an achromatic coupler, the connection pigtails of which can becoupled with low insertion loss to system fibers. Yet another object isto provide an achromatic coupler, wherein the feature or parameter thatmodifies the propagation constant β of the non-standard fiber hasnegligible effect on the fiber mode field diameter. A further object isto provide a reproducible method of making achromatic overclad fiberoptic couplers.

The achromatic coupler of the invention comprises an elongated body ofmatrix glass having a refractive index n₃. The body includes two opposedends and a midregion. A plurality of optical fibers extendslongitudinally through the body, each of the fibers comprising a core ofrefractive index n₁ and a cladding of refractive index less than n₁ butgreater than n₃. The refractive index n₂ of the cladding of the firstfiber is different from the refractive index n₂ ' of the cladding of thesecond fiber by such an amount that the value of Δ_(clads) is greaterthan zero but less than 0.03%. The fibers are fused together along withthe midregion of the matrix glass. The diameters of the optical fibersin the central portion of the midregion are smaller than the diametersthereof at the ends of the body, whereby a portion of the optical powerpropagating in one of the fibers couples to the other of the fibers.

The value of Δ_(clads) is preferably greater than 0.005%. To form acoupler that is capable of coupling about 50% of the power from thefirst fiber to the second fiber at a predetermined wavelength, the valueof Δ_(clads) is preferably less than 0.02%. The refractive index n₃ ispreferably such that Δ₂₋₃ is greater than 0.4%. Couplers made inaccordance with the invention have exhibited an insertion loss less than4 dB in each leg thereof over a 300 nm range of wavelengths up to 1565nm.

The achromatic fiber optic coupler of the present invention is formed byinserting into a glass tube at least a portion of each of a plurality ofoptical fibers so that the portions occupy the midregion of the tube.Each of the fibers comprises a core of refractive index n₁ and acladding of refractive index less than n₁, the refractive index n₂ ofthe cladding of the first fiber being different from the refractiveindex n₂ ' of the cladding of the second fiber. The difference betweenn₂ and n₂ ' is such that the value of Δ_(clads) is greater than zero butless than 0.03%. The midregion of the tube is collapsed onto fibers, andthe central portion of the midregion is stretched until a predeterminedcoupling occurs between the fibers.

The step of stretching may comprise providing relative movement betweenthe ends of the tube, and varying the rate at which the relativemovement occurs. The stretching rate can vary continuously, or thevariation can occur in descrete steps. One stretching operation can stopafter a predetermined coupling is achieved; thereafter, stretching canoccur at a second stretch rate.

The stretching operation can be stopped before a predetermined couplingis achieved; thereafter, the central portion of the tube midregion canbe reheated, and the central portion of the tube midregion can again bestretched. The reheat temperature is preferably lower than thetemperature to which the tube is initially heated. The last employedstretch rate may be lower than the first stretch rate.

In an embodiment wherein a first fiber extends from both ends of thetube, and a second fiber extends from only the second end of the tube,the coupler preform can be stretched until some coupling begins to occurbetween the fibers. Detector can be connected to the ends of the firstand second fibers which extend from the second end of the tube. Thecoupled power is employed to maximize the power coupled from the secondfiber to its respective detector. The ratio of the optical power coupledto the two detectors is used to generate the signal which stops thestretch operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an overclad coupler.

FIGS. 2 and 3 are graphs of output voltage v. stretching time forcouplers having two different overclad refractive indices.

FIG. 4 is a graph of the coupled power slope (centered around 1310 nm)plotted as a function of Δ_(clads).

FIG. 5 shows theoretical spectral response curves for single-window anddouble-window achromatic couplers wherein Δ_(clads) is 0.005%.

FIG. 6 is a graph which schematically illustrates the temporal variationin percent coupled power during the stretching of couplers havingdifferent values of Δ_(clads)

FIG. 7 is a graph illustrating non-uniform stretch rates.

FIG. 8 is a graph illustrating the effect of chlorine on Δ_(clads).

FIG. 9 is a refractive index profile of a non-standard fiber employed inthe coupler of the invention.

FIGS. 10-12 illustrate achromatic couplers having more than two ports atone end thereof.

FIG. 13 is a cross-sectional view of a capillary tube after opticalfibers have been inserted therein.

FIGS. 14 and 15 are schematic illustrations of two steps during theformation of an antireflection termination on a fiber.

FIG. 16 is a schematic illustration of an apparatus for collapsing acapillary tube and stretching the midregion thereof.

FIG. 17 is a partial cross-sectional view illustrating the collapse ofthe glass tube around the fibers to form a solid midregion.

FIG. 18 is a partial cross-sectional illustration of a fiber opticcoupler after it has been drawn down and sealed at its ends.

FIG. 19 is a graph illustrating the spectral insertion loss curves foran achromatic coupler produced by the method of Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings are not intended to indicate scale or relative proportionsof the elements shown therein.

Referring to FIG. 1, each of the optical fibers F₁ and F₂ has a core ofrefractive index n₁ surrounded by cladding of refractive index lowerthan n₁. The claddings of fibers F₁ and F₂ have different refractiveindices n₂ and n₂ ', respectively, the values of which are such that thepropagation constants of those fibers differ to the extent necessary toprovide achromaticity.

A coupler preform is formed by threading fibers F₁ and F₂ through glassoverclad tube 0, the refractive index n₃ of which is less than therefractive indices of the fiber claddings. Whereas those portions of thefibers extending from the tube preferably have protective coatingmaterial (not shown in this illustrative embodiment), those portionsthereof within the tube have no coating. The original diameter of thetube is d₁. The midregion of the coupler preform is evacuated and heatedto collapse it onto the fibers. The tube is reheated and the endsthereof are pulled in opposite directions to increase the tube lengthand reduce its diameter. The combined rate at which the two tube endsmove away from each other constitutes the stretch rate. The centralportion of the stretched midregion constitutes neckdown region N ofdiameter d₂ where the fiber cores are sufficiently closely spaced for asufficiently long distance z to effect the desired couplingtherebetween. Region N is illustrated as having a constant diameter eventhough a slight taper exists therein, whereby the longitudinal center ofsection N exhibits the minimum diameter. Draw ratio R, which is equal tod₁ /d₂, is a critical parameter in determining the opticalcharacteristics of the particular device being made. A preferred rangeof draw ratios for achromatic overclad couplers is between about 3:1 and10:1 depending upon the value of Δ_(clads) and the amount of power to becoupled. Tapered regions T connect the neckdown region with theunstretched end regions of tube 0. The duration of the heating periodfor the stretch step is shorter than that for the tube collapse step;only the central portion of the midregion is stretched.

It is conventional practice to monitor output signals to control processsteps in the manufacture of optical devices as evidenced by U.S. Pat.Nos. 4,392,712 and 4,726,643, 4,798,436, U.K. Patent Application No. GB2,183,866 A and International Publication No. WO 84/04822. Furthermore,computers are often employed in feedback systems which automaticallyperform such monitor and control functions. A suitably programmedDigital PDP 11-73 micro-computer can be utilized to perform thesefunctions. During the tube collapse and stretch steps, the ends of thetube are affixed to computer controlled stages. The amount of stretchingto which the tube must be subjected to achieve given characteristics isinitially determined by injecting light energy into the input fiber of acoupler preform and monitoring the output power at one or more of theoutput fibers during the stretch operation. If a 2x2 coupler is beingformed, a light source can be connected to an input end of the first andsecond fibers, and a detector can be aligned with the output endsthereof, the fibers being manipulated to maximize the output powercoupled to each detector. During stretching, the input end of only thefirst fiber is connected to a source, and the output ends of both fibersare monitored. The detection of a predetermined ratio of powers at theoutputs of the first and second fibers can be used as an interrupt tocause the computer controlled stages to stop pulling the sample. If a1x2 coupler is being formed, the second fiber cannot be accuratelypositioned with respect to certain detectors until some light is coupledthereto from the first fiber. An achromatic coupler can be made bymonitoring only the output from the first fiber. When the output fromthe first fiber drops to a predetermined value, the system is instructedto stop stretching. An alternative procedure for monitoring 1x2 couplersis described below.

After having determined the proper stretching distance to achievepredetermined coupling characteristics, the apparatus can be programmedto move the stages that proper stretching distance during thefabrication of subsequent couplers that are to have said predeterminedcharacteristics. The timing sequences that have been used in thefabrication of a particular type of coupler can be entered in a separatemultiple command file that the computer recalls at run-time. Thecollapse and stretch steps that are required to make that particularcoupler can be executed in succession by the computer on each couplerpreform to reproducibly manufacture couplers. The process parametersthat can be controlled by the computer to ensure coupler reproducibilityare heating times and temperatures, gas flow rates, and the rate orrates at which the stages pull and stretch the coupler preform.

If the device that is being made is a 3 dB coupler, for example, thestretching operation is not stopped when the output power from the twofibers is equal. Various parts of the system exhibit momentum, wherebystretching of the coupler preform continues after the stage motors areinstructed to stop. The coupling ratio therefore changes after thestopping signal is generated. Also, the coupling characteristics maychange as a newly formed coupler cools down. Experiments can beperformed on a particular type of coupler to determine that couplingratio which must be used to generate the interrupt signal in order toachieve a predetermined coupling ratio after the device cools.

Following are examples of the various stretching operations that can beperformed.

A. Heat the coupler preform, and stretch it at a single rate until apredetermined coupling has been achieved.

B. After subjecting the coupler preform to a single heating step,stretching it at differing stretch rates until a predetermined couplinghas been achieved. Two or more discrete stretch rates could be employed,or the stretch rate could continually vary with respect to time. Thisstretching technique has been employed to tune the power transfercharacteristic, i.e. the amount of power transfered from the input fiberto the output fiber during the first power transfer cycle of the couplerpreform stretching operation.

C. Heat the coupler preform and perform a first stretch which does notachieve the predetermined coupling; reheat the resultant device andperform a second stretch. The heat and reheat steps may be performed ata single temperature or at different temperatures. The first and secondstretch steps can be done at the same stretch rate or at differentstretch rates. More than two heat and stretch steps could be performed.

A species of stretching embodiment C is especially useful in theformation of 1x2 couplers. The stretching operation is temporarilyterminated after some minimal amount of power has been coupled to thesecond fiber. For example, stretching could be stopped after the couplerpreform has been stretched some predetermined distance, such as between90% and 99% of the total distance required to achieve the final couplingratio. The second fiber can be connected to a detector, and the powercoupled to that detector can be maximized. Thereafter, a secondstretching operation can be initiated, the interrupt signal being basedon the ratio of the two output signals. The second stretching operationis preferably conducted at a final stretch rate that is lower than theinitial stretch rate. Also, during the second stretch, it is preferableto employ a flame which has a lower temperature and/or which is lessfocussed than the flame employed during the first stretch.

Tube 0 can be characterized by the symbol Δ₂₋₃, the value of which isobtained by substituting n₂ and n₃ into equation (1). Commerciallyavailable single-mode optical fibers usually have a value of n₂ that isequal to or near that of silica. If silica is employed as the base glassfor the tube, a dopant is added thereto for the purpose of decreasingthe tube refractive index n₃ to a value lower than n₂. In addition tolowering the refractive index of the tube, the dopant B₂ O₃ alsoadvantageously lowers the softening point temperature thereof to a valuelower than that of the fibers. This enhances to a certain extent thecollapsing of the tube onto the fibers; the tube glass flows around thefibers without distorting their shape. For certain purposes it may bedesirable to employ a tube glass that is sufficiently hard that the tubeslightly flattens the fibers as it forces the fibers together. Fluorinecan also be employed to lower the tube refractive index. Suitable tubecompositions are SiO₂ doped with 1 to 25 wt. % B₂ O₃, SiO₂ doped with0.1 to approximately 2.5 wt. % fluorine, and SiO₂ doped withcombinations of B₂ O₃ and fluorine. When Δ₂₋₃ is below about 0.2%, theamount of B₂ O₃ in a silica tube is insufficient to soften the tubeglass, whereby it excessively deforms the fibers during the collapsestep. The value of Δ₂₋₃ for standard couplers has therefore usually beenbetween 0.26% and 0.35%. Suitable achromatic overclad couplers have beenmade from preforms comprising tube and fibers exhibiting refractiveindex values such that Δ₂₋₃ is within that range. However, processreproducibility is enhanced by employing preforms having Δ₂₋₃ valuesabove that previously employed range.

To demonstrate the effect of the overclad glass, reference is made toFIGS. 2 and 3 which are plots of the voltage from a detector connectedto the output end of the input fiber during the manufacture of 1x2couplers wherein the values of Δ₂₋₃ are 0.36% and 0.48%, respectively.Referring to FIG. 2, the output is initially highest at point a sincecoupling has not yet occurred. As the stretching process is initiatedand power begins to couple, the power remaining in the input fiberbegins to decrease at some point in time after point a. At point b, thedetected power is such that the computer controlled stages areinstructed to stop moving. A few microseconds later, the stretching stepis terminated (point c), and the finished coupler begins to cool. Duringcooling, the coupled power begins to vary until it finally stabilizes atpoint d when it is sufficiently cool that there is no further change instress or refractive index within the coupler. It is possible, byexperimentation, to form a coupler, the 3 dB point of which is within 10nm of the desired wavelength by causing the stretching operation to stopat some predetermined coupling other than 50%.

The meandering of the coupling ratio upon cooling of the coupler can beessentially eliminated by utilizing overclad tubes of sufficiently highrefractive index that the value of Δ₂₋₃ is greater than about 0.4%. Thisstabilizing effect is illustrated in FIG. 3 wherein reference letterssimilar to those of FIG. 2 are represented by primed reference numerals.The amount of coupled power begins to decrease at point a', the computercontrolled stages stop moving at point b', and the stretching step isterminated at point c'. During cooling, the coupled power varies onlyslightly until it stabilizes at point d'. After the stretching hasstopped (points c and c'), the coupled power will more predictably reachpoint d' than point d.

A theoretical analysis was made of 3 dB couplers of the type wherein Δβwas obtained by fiber cladding index difference. Coupled mode theory wasused to model the behavior of the achromatic couplers [A. W. Snyder andJ. D. Love, Optical Waveguide Theory, Chapman and Hall, New York, 1983].In accordance with this theory, the mode field of the overclad coupleris assumed to be a linear combination of the fundamental modes ψ₁ and ψ₂of each of the fibers F₁ and F₂ in the absence of the other fiber, i.e.with the fiber surrounded by overclad index n₃ only. The mode fields andpropagation constants can be determined exactly for such a structure [M.J. Adams, An Introduction to Optical Waveguides].

The coupling constant which describes the optical coupling between thetwo cores can then be written as an overlap integral:

    C=∫ψ.sub.1 (r)ψ.sub.2 (r')(n-n')dA            (2)

In this equation, ψ₁ and ψ₂ are the mode fields of the two cores, r andr' are the radial distances from the center of the cores of fibers F₁and F₂, respectively, n is the index structure of the entire coupler, n'is the index structure with the core of F₁ replaced by overcladdingmaterial of index n₃, and the integral is over the entire cross-sectionof the coupler (but n-n' is only non-zero over the core and cladding offiber F₁). The mode fields are assumed to be normalized in thisequation, i.e. the integrals ∫ψ₁ ₂ dA and ∫ψ₂ ² dA both equal 1.

While these are tapered devices, their behavior is adequately modeled byassuming a constant draw ratio over a given coupling length., with nocoupling outside this length, i.e. assuming that the diameter of regionN of FIG. 1 is constant over the entire length z. This approximationworks well since the coupling constant is a rapidly increasing functionof draw ratio, and thus the behavior of a coupler is dominated by thebehavior at the highest draw ratio. Using this approximation, with thepower launched into core 1, then, as a function of z, the length alongthe coupler axis, the power in the two cores is given by

    P.sub.1 (z)=1-F.sup.2 sin.sup.2 (Cz/F)                     (3)

and

    P.sub.2 (z)=F.sup.2 sin.sup.2 (Cz/F)                       (4)

where the factor F is given by ##EQU1## where β₁ and β₂ are thepropagation constants of fibers F₁ and F₂, respectively.

Optimal achromatic performance was defined, for a single-window devicehaving a center wavelength of 1310 nm and a width of 50 nm, as being thepoint where

    P.sub.2 (1297.5 nm)=P.sub.2 (1322.5 nm)=0.5                (6)

The achromaticity was defined as ##EQU2##

The coupled mode model was used to determine a suitable range ofΔ_(clads). Most of the assumptions which were made concerning couplerparameters are based on work done on standard overclad couplers. FiberF₁ was assumed to be a standard 125 μm outside diameter single-modefiber having a core radius of 4 μm. The core and cladding refractiveindices n₁ and n₂ were assumed to be 1.461000 and 1.455438,respectively. It was assumed that fiber F₂ was identical to fiber F₁except that the cladding index n₂ ' was greater than n₂. The value ofΔ₂₋₃ was assumed to be 0.3%. In order to determine the combination ofdraw ratio and length z for which achromaticity was best, P₂ wascalculated at the appropriate wavelengths for a range of draw ratios.The combination of draw ratio and coupling length z which satisfiedequation (6) was determined, and then the achromaticity (the variationin coupled power in percent per nanometer) calculated for thatcombination.

As shown in FIG. 4, the theoretical analysis revealed that the variationin coupled power (at 1310 nm) increases as the value of Δ_(clads)decreases. This is in accordance with the expected relationship wherebycoupler achromaticity decreases as the difference between the fiberpropagation constants decreases. The relationship shown in FIG. 4 is forcouplers having a Δ₂₋₃ value of 0.3%. For couplers having greater valuesof Δ₂₋₃, the curve is displaced toward higher values of variation inpercent coupled power. When the value of Δ_(clads) is less than 0.005%,the variation in percent coupled power rapidly increases. Theachromaticity therefore rapidly decreases at values of Δ_(clads) belowthis value. Also, as the value of Δ_(clads) decreases below 0.005%, therequired length of the neckdown region increases to such an extent thatthe resultant achromatic coupler would be impractical in that it wouldbe undesirably long and would be difficult to make.

FIG. 5 shows the theoretical relationship of coupled power with respectto wavelength for both single-window and double-window couplers, withΔ_(clads) =0.005% and Δ₂₋₃ =0.3%. The value of d₁ /d₂ is 6.6 for thesingle-window device as determined by requiring equation (6) to besatisfied. The value of d₁ /d₂ is 6.2 for the double-window device asdetermined by requiring an analogous equation to be satisfied for thewavelengths 1310 nm and 1550 nm.

Whereas the model indicated that a draw ratio of about 6:1 would beneeded to form a coupler wherein Δ_(clads) is 0.005%, 3 dB achromaticcouplers having low values of Δ_(clads) have been made having drawratios as low as about 3.5:1. The draw ratio can be even lower for taps(less than 50% coupling) since less stretching is required. As the valueof Δ_(clads) increases, the draw ratio must increase in order to achievethe desired coupling ratio. Although FIG. 4 would seem to suggest that aΔ_(clads) value of 0.025% would be desirable from the standpoint ofproviding very good achromaticity, such a coupler is difficult to makesince the draw ratio required to make it is around 10:1. Also, forreasons discussed below, the coupled power at higher values of Δ_(clads)may be inadequate to achieve the desired coupling ratio.

While a coupler preform is being stretched to form a coupler, thediameter of neckdown region N becomes smaller with increasing time. FIG.6 shows that the coupled power varies during the stretching process. Thecurves of FIG. 6 do not bear an exact relationship with respect to oneanother; rather, it is intended that they qualitatively illustrate therelative relationship between the temporal coupled power curves ofcouplers having different Δ_(clads) values. During the stretching of astandard coupler (Δ_(clads) =0), the coupled power relatively quicklyreaches 50% and eventually reaches almost 100%. During the stretching ofdevices having greater values of Δ_(clads) , greater time periods arerequired to achieve 50% coupling, and the maximum possible amount ofcoupled power decreases. For a given set of stretching conditionsincluding rate of stretch, temperature of the coupler preform, and thelike, there will be a value of Δ_(clads) for which the coupled powerjust reaches 50% on the first peak of the coupled power curve. For agiven set of draw conditions, this value of Δ.sub. clads is shown inFIG. 6 to be 0.015%. For higher values of Δ_(clads) such as 0.025%, thefirst power transfer peak of the coupled power curve cannot provide 50%coupling. However, it can be seen that a device for coupling less thanhalf the input power, for example a 10% tap, might easily be made bystretching a coupler preform having a Δ_(clads) value of 0.025% untilthe coupled power is 10%, a value that can be attained on the firstpeak.

The curves of FIG. 6 are not continued in time any further than theextent necessary to illustrate the specific point being discussed. Thefirst power transfer peak is shown for couplers wherein Δ_(clads) is0.015 and 0.025. Subsequent power transfer peaks are not shown. However,if the coupler preforms were stretched for longer periods of time, thecoupled power would continue to oscillate between zero and some maximumvalue, the period of each subsequent oscillation being narrower than theprevious one. If the curves representing couplers having Δ_(clads)values of 0 and 0.005 were continued in time, they would experiencesimilar oscillations in coupled power. The relationship between coupledpower and coupling length (which is a function of stretching time) overa plurality of coupled power peaks is graphically illustrated in theaforementioned U.S. Pat. No. 4,798,436.

It is assumed that curve t (Δ_(clads) =0.025%) is for a stretchingoperation wherein the coupler preform is heated once and stretched at asingle rate. If all other conditions remain the same, the power transfercurve can be

displaced upwardly to curve t' (toward greater power transfer) bystretching the coupler preform at more than one stretch rate asillustrated in FIG. 7. By way of example only, FIG. 7 illustrates astretch technique involving stretching at two discrete rates (curves s₁and s₂) and a technique wherein the stretch rate varies continually withrespect to time (curve s'). In accordance with a specific embodimentdepicted in FIG. 7, the coupler preform is heated and stretched 0.2 cmat a stretch rate of 0.95 cm/sec, the stretch rate abruptly decreasingto 0.45 cm/sec while the coupler preform is stretched an additional 0.55cm.

For certain stretching conditions, including a Δ_(clads) value of about0.025% or higher, a subsequent power transfer peak such as the thirdpeak might be required to reach the desired coupling value, e.g. 50%.Since the third peak is much narrower than the first, the stretchingoperation must be stopped at precisely the right time in order toachieve the desired coupling ratio. If stretching is continued for onlya short additional length of time, the neckdown ratio may changesufficiently to cause the coupled power to drastically decrease. It isdifficult to draw such a coupler when output power is being monitored tostop the draw, and it is almost impossible to make such a coupler bydrawing to a predetermined length. Furthermore, the achromaticitybecomes degraded when the coupler has to be stretched beyond the firstpower transfer peak. For the aforementioned reasons, the maximumpreferred value of Δ_(clads) for 3 dB couplers is about 0.025% and themaximum value of Δ_(clads) for a power tap is about 0.03%.

In view of the value of Δ_(cores) that was required for the achromaticfused fiber coupler taught in the aforementioned Sweeney et al. patent,the above-defined range of Δ_(clads) that is suitable for achromaticoverclad couplers is unexpectedly low. It appears that the presence ofthe overclad tube enables the achievement of achromaticity withrelatively small values of Δ_(clads) and that a value of Δ_(clads)larger than 0.03% would be required if no overclad tube were employed,i.e. for a fused fiber coupler.

The low range of values of Δ_(clads) that was determined by theaforementioned model has been verified by experimental results. Whencouplers were formed having values of Δ_(clads) below about 0.005%, Δβwas so insignificant that coupling behavior approached that of astandard coupler. Couplers having a Δ_(clads) value in the range ofabout 0.015% exhibited an insertion loss of less than 4 dB in each legthereof over a 300 nm range of wavelengths up to 1565 nm.

A number of advantages result from the unexpectedly low values ofΔ_(clads). Couplers having low Δ_(clads) values can be connected withlow loss into the system. One of the fibers can be a standardsingle-mode fiber. To provide a Δ_(clads) value of 0.015%, for example,the cladding index of the other fiber (or non-standard fiber) needdiffer from that of the standard fiber by only 0.00022. Suchnon-standard fiber exhibits substantially the same mode field diameteras the standard fiber. Since the diameters of both fibers aresubstantially identical, the non-standard fiber, as well as the standardfiber, can be connected to the system fibers with low loss.

The required value of Δ_(clads) can be obtained by adding a dopant tothe cladding of only one of the fibers or by adding different amounts ofthe same or different dopants to the claddings of the two fibers. Forexample, the cladding of one fiber could consist of silica and that ofthe other could consist of silica doped with fluorine or B₂ O₃ to lowerthe refractive index or silica doped with chlorine, GeO₂ or the like toincrease the refractive index.

The process of making the non-standard fiber is facilitated by the lowvalue of Δ_(clads) that is required to form an achromatic coupler. Whenadded to silica, commonly employed dopants such as B₂ O₃, fluorine, GeO₂and the like have a relatively large effect on refractive index. It istherefore difficult to deliver such dopants in the small, preciselycontrolled amounts that are necessary to change the refractive index ofthe base glass to an extent sufficient to provide a Δ_(clads) valuebetween 0.005% and 0.03%. It has been found that chlorine has asufficient effect on the refractive index of silica that it can be usedas a dopant in the cladding of the non-standard fiber. Since the changein refractive index per weight percent dopant in silica is much less forchlorine than for conventional dopants such as B₂ O₃, fluorine, GeO₂ andthe like, chlorine can be used to provide precisely controlledrefractive index values that are only slightly higher than that of thesilica to which the chlorine is added. Furthermore, the use of chlorinesimplifies the process of making the non-standard fiber since it isconventionally employed for drying purposes. Sufficient amounts ofchlorine can simply be added to the cladding region of the non-standardfiber in conjunction with the drying/consolidation process.

The standard fiber can be made by a conventional process, such as thatdisclosed in U.S. Pat. No. 4,486,212, which is incorporated herein byreference. Briefly, that process consists of forming on a cylindricalmandrel a porous preform comprising a core region and a thin layer ofcladding glass. The mandrel is removed, and the resultant tubularpreform is gradually inserted into a consolidation furnace muffle, themaximum temperature of which is between 1200° and 1700° C. andpreferably about 1490° C. for high silica content glass. The temperatureprofile of the muffle is highest in the central region as taught in U.S.Pat. No. 4,165,223, which is incorporated herein by reference. Chlorine,which is present in the minimum concentration that is required toachieve drying, may be supplied to the preform by flowing into thepreform aperture a drying gas consisting of helium and about 5 volumepercent chlorine. The end of the aperture is plugged to cause the gas toflow through the preform pores. A helium flushing gas is simultaneouslyflowed through the muffle.

The resultant tubular glass article is stretched in a standard drawfurnace while a vacuum is applied to the aperture to form a "core rod"in which the aperture has been closed. A suitable length of the rod issupported in a lathe where particles of silica are deposited thereon.The resultant final preform is gradually inserted into the consolidationfurnace where it is consolidated while a mixture of 99.5 volume percenthelium and 0.5 volume percent chlorine is flowed upwardly therethrough.The resultant glass preform is drawn to form a step-index, single-modeoptical fiber, the entire cladding of which comprises silica doped witha residual amount of chlorine. When the cladding is consolidated in astandard downfeed consolidation furnace, as described above, about0.04-0.06 wt. % chlorine is normally present in the fiber cladding.

The non-standard fiber can be made by a process which is initiallyidentical to the process by which the standard fiber is made. Forexample, the core rod, which consists of a solid glass rod of coreglass, that is optionally surrounded by a thin layer of silica claddingglass, is initially formed. A porous layer of silica particles isdeposited on the rod, and the porous layer is consolidated in anatmosphere containing an amount of chlorine greater than that whichwould be necessary for drying purposes. The chlorine concentration inthe consolidation furnace is controlled to provide the desired value ofΔ_(clads). The amount of chlorine that is incorporated into the baseglass depends upon various process conditions such as the maximumtemperature and temperature profile of the consolidation furnace, theconcentrations of chlorine and oxygen therein and the rate of insertionof the preform into the furnace. The porosity and composition of thepreform would also affect the final chlorine concentration. A graph suchas that shown in FIG. 8 can be generated for a given standard fiber. Forthe specific relationship shown in FIG. 8, the standard fiber claddingcontained about 0.05 wt. % chlorine. Therefore, about 0.2 wt. % chlorineshould be incorporated into the cladding of the non-standard fiber toachieve a Δ_(clads) value of 0.015%. This chlorine concentration isdetermined by reading from the graph of FIG. 8 the incremental increasein chlorine content for the desired value of Δ_(clads) and adding 0.05wt. %. If desired, both fibers could be of the non-standard type, i.e.both could contain more chlorine than standard, commercially availablefibers. For example, a Δ_(clads) value of 0.015% could also be obtainedby utilizing fibers, the claddings of which contain 0.10 wt. % and 0.23wt. % chlorine.

If the non-standard fiber is made by initially forming a core rodcomprising core glass surrounded by a thin layer of cladding glass(containing a small amount of residual chlorine) and the outer claddingglass is doped with a larger amount of chlorine, the refractive indexprofile of the resultant fiber would appear as illustrated in FIG. 9.The radii of the various layers of a standard fiber might be 4 μm coreradius r₁, 10.5 μm inner cladding radius r₂ and 62.6 μm outer radius r₃.Because of the small area of the inner cladding layer, the refractiveindex of that layer need not be taken into consideration when specifyingthe cladding refractive index. That is, the effective refractive indexof the entire cladding beyond radius r₁ is essentially the same as thatof the layer between r₂ and r₃.

It is noted that attempts have been made by certain fiber manufacturersto reduce the amount of chlorine in optical fibers in order to lower theattenuation (see Japanese Kokai No. 63/285137). If one fiber had a puresilica cladding (by removing the chlorine therefrom) about 0.13 wt. %chlorine would be needed in the other fiber to achieve a Δ_(clads) valueof 0.015%. However, it has been found that the presence of chlorine inthe short lengths of coupler fibers has little or no effect on couplerloss. The additional step of removing chlorine from coupler fibers wouldtherefore be an unnecessary expense.

Whereas 2x2 couplers are illustrated in FIG. 1, this invention alsoapplies to other configurations. An NxN coupler (N>1) can be formed forthe purpose of coupling one fiber to N fibers. A 1x2 coupler isdescribed in the specific embodiment. More than 2 fibers can be joinedat their waists to form an NxN coupler. Sometimes, one or more fibersare severed from one end of an NxN coupler so that a plurality offibers, unequal in number, extend from opposite ends of the coupler. Theembodiments of FIGS. 10-12 are schematic illustrations of coupledfibers, the overclad tubing glass having been omitted for simplicity.The presence of an overclad glass is indicated by the symbol n₃ adjacentthe fibers. In the 1x3 coupler of FIG. 10, standard fiber S is coupledto two non-standard fibers S⁺ and S⁻. The refractive index of thecladding of fiber S⁺ is negative with respect to the cladding of fiberS, whereby the value of Δ_(clads) of fiber S⁺ with respect to fiber S ispositive. The refractive index of the cladding of fiber S⁻ is such thatthe value of Δ_(clads) of fiber S⁻ with respect to fiber S is negative.

In the 1x4 embodiment of FIGS. 11 and 12, the refractive index of thecladding glass of fibers S⁺ is such that the value of Δ_(clads) offibers S⁺ with respect to fiber S is positive. FIG. 12 shows that fibersS⁺ are preferable equally spaced around fiber S.

Whereas the preferred manufacturing technique results in a couplerhaving optical fiber pigtails extending therefrom, the invention alsoapplies to overclad couplers of the type wherein the fibers extendthrough the elongated matrix glass body but end flush with the bodyendface. Methods of making such a coupler are disclosed in U.S. Pat.Nos. 4,773,924 and 4,799,949. Briefly, the method comprises inserting aplurality of optical fiber preform rods into a glass tube, heating andstretching the resultant preform to form a glass rod which is thensevered into a plurality of units. Heat is applied to the central regionof each unit, and the central region is stretched to form a taperedregion as described herein.

A method of making 1x2 achromatic 3 dB fiber optic couplers isillustrated in FIGS. 13-18. A glass capillary tube 10 having a 3.8 cmlength, 2.8 mm outside diameter, and 270 μm longitudinal aperturediameter was secured by chucks 32 and 33 of the apparatus of FIG. 16.Tube 10, which was formed by a flame hydrolysis process, consisted ofsilica doped with about 6 wt. % B₂ O₃ and about 1 wt. % fluorine.Tapered apertures 12 and 13 were formed by flowing the gas phase etchantNF₃ through the tube while uniformly heating the end of the tube.

Coated fibers 17 and 18 comprised 125 μm diameter single-mode opticalfibers 19 and 20 having a 250 μm diameter urethane acrylate coatings 21and 22, respectively. Both fibers had a 8 μm diameter core of silicadoped with 8.5 wt. % GeO₂. The cutoff wavelengths of the fibers arebelow the operating wavelength of the coupler. If, for example, theminimum operating wavelength is 1260 nm, the cutoff wavelengths of thefibers are selected to be between 1200 nm and 1250 nm. All chlorineconcentrations were measured by microprobe techniques. The initial stepsof the processes of making both fibers was the same; these steps are setforth above in conjunction with a discussion of U.S. Pat. No. 4,486,212.A first layer of glass particles comprising SiO₂ doped with 8.5 wt. %GeO₂ was deposited on a mandrel, and a thin layer of SiO₂ particles wasdeposited on the first layer. The mandrel was removed, and the resultantporous preform was gradually inserted into a furnace having an aluminamuffle where it was dried and consolidated. During this process, an gasmixture containing 65 sccm (standard cubic centimeter per minute)chlorine and 650 sccm helium flowed into the center hole where themandrel had been removed. A flushing gas containing 40 slpm (liter perminute) helium and 0.5 slpm oxygen flowed upwardly from the bottom ofthe muffle. The aperture was evacuated, and the lower end of the tubularbody was heated to 1900° C. and drawn at a rate of about 15 cm/min toform a 5 mm solid glass rod. The rod was severed to form sections, eachof which was supported in a lathe where it functioned as a mandrel uponwhich SiO₂ cladding soot was deposited to form a final porous preform.

a. Forming a Standard Fiber

One final porous preform was gradually inserted into the alumina muffleof a consolidation furnace having a maximum temperature of 1490° C. Agas mixture containing 40 slpm helium, 0.5 slpm chlorine and 0.5 slpmoxygen flowed through the muffle. The porous preform was consolidated toform a draw blank, the outer cladding of which had the same compositionas the inner cladding layer, i.e. SiO₂ doped with about 0.05 wt. %chlorine. The tip of the draw blank was heated to about 2100° C., and astandard optical fiber was drawn therefrom, the fiber being coatedduring drawing. The fiber had an 8 μm diameter core and a 125 μmdiameter homogeneous cladding layer of silica containing about 0.05 wt.% chlorine as a residual from the drying process.

b. Forming a Non-Standard Fiber

Another final porous preform was gradually inserted into a consolidationfurnace having a sileca muffle. The maximum temperature of 1450° C. Theporous preform was subjected to an upwardly flowing gas mixturecontaining about 2 slpm helium and 0.6 slpm chlorine. The porous preformwas consolidated to form a draw blank, the outer cladding of whichconsisted of SiO₂ doped with about 0.2 wt. % chlorine. The resultantnon-standard fiber was similar to the standard fiber except that it hada 10.5 μm diameter inner cladding region containing about 0.05 wt. %chlorine and an outer, 125 μm diameter cladding region containing about0.2 wt. % chlorine. The refractive indices of the claddings of thisfiber and the standard fiber were such that the value of Δ_(clads) was0.015.

The standard and non-standard fibers were interchangable in thefollowing process.

A 6 cm long section of coating was removed from the end of a 1.5 meterlength of coated fiber 18. A flame was directed at the center of thestripped region of fiber, and the end of the fiber was pulled andsevered to form a tapered end (FIG. 14). The fiber end remote from thetapered end was connected to a reflectance monitoring apparatus. Thetapered end was moved slowly along its longitudinal axis to the right(as shown in FIGS. 14 and 15 wherein only the bright, central portion 23of the flame is illustrated). As the tip of fiber 20 was heated by flame23 of burner 24', the glass receded and formed rounded endface 25 (FIG.15), the diameter of which was preferably equal to or slightly smallerthan the original uncoated fiber diameter. A current specification forthe reflected power is -50 dB. The resultant length of uncoated fiberwas about 2.9 cm.

Tube 10 was inserted through ring burner 34 (FIG. 16) and was clamped todraw chucks 32 and 33. The chucks were mounted on motor controlledstages 45 and 46 which were controlled by a computer. Approximately 3.2cm of coating was stripped from the central region of a 3 meter lengthof fiber 17. The uncoated sections of fibers 17 and 18 were wiped, and asmall amount of ethyl alcohol was squirted into the tube to temporarilylubricate the fibers during the insertion process.

Coated fiber 17 was inserted through aperture 11 until its uncoatedportion was situated below tube end 15. The uncoated portion of coatedfiber 18 was held adjacent the uncoated portion of coated fiber 17, andboth were moved together toward tube end 14 until the coating endregions become wedged in tapered aperture 13. The uncoated portion ofcoated fiber 17 was then disposed intermediate end surfaces 14 and 15,the uncoated portion of coated fiber 17 preferably being centered withinaperture 11. End 25 of fiber 18 was located between midregion 27 and end14 of tube 10. The fibers were threaded through the vacuum attachments41 and 4140 , which were then attached to the ends of preform 31.Referring to FIG. 13, vacuum attachment 41 was slid over the end of tube10, and collar 39 was tightened, thereby compressing 0-ring 38 againstthe tube. Vacuum line 42 was connected to tube 40. One end of a lengthof thin rubber tubing 43 was attached to that end of vacuum attachment41 opposite preform 31; the remaining end of the tubing extendingbetween clamp jaws 44. Upper vacuum attachment 41' was similarlyassociated with line 42', tubing 43' and clamp jaws 44'. The coatedportions of the fibers extended from tubing 43 and 43'.

Vacuum was applied to the lower portion of coupler preform 31 byclamping jaws 44 on tubing 43 while the upper vacuum attachment wasconnected to a source of nitrogen to purge the aperture contents. Jaws44' were then clamped against tubing 43' to apply vacuum to the upperportion of preform 31.

The upper end of fiber 17 was connected to a monochromater coupled to awhite light source. The monochromater was adjusted so that the fiber wasprovided with a beam 1310 nm light. The lower end of fiber 17 wasconnected to a detector which formed a part of the system that controlsthe movement of chucks 32 and 33.

With a vacuum of 10 inches (25.4 cm) of mercury connected to the tubeaperture, ring burner 34 was ignited. The apparatus located above ringburner 34 was protected by heat shield 35. Flames of about 1800° C. weregenerated by supplying gas and oxygen to the burner at rates of 0.8 slpmand 0.85 slpm, respectively. The flame from ring burner 34 heated tube10 for about 25 seconds. The matrix glass collapsed onto fibers 19 and20 as shown in FIG. 17. Midregion 27, the central portion of which formsthe coupling region of the resultant coupler, became a solid regionwherein substantially the entire lengths of fibers 19 and 20 were inmutual contact.

After the tube cooled, the flow rates of both the gas and oxygen wereincreased to 0.9 slpm, and the burner was reignited. Flames having atemperature of about 1900° C. heated the center of the collapsed regionto the softening point of the materials thereof. After 12 seconds, thesupply of oxygen to burner 34 was turned off. Stages 45 and 46 werepulled in opposite directions at a combined rate of 2.5 cm/sec until thecentral portion of midregion 27 was stretched 1.46 cm. The flame becameextinguished after the stretching operation. This increase in length wasjust short of the length to which coupler preform 31 would have had tobe stretched in order to have achieved achromaticity in a singlestretching operation. A sufficient amount of power began to couple tofiber 18 to enable the end of that fiber to be connected to a detector,and the power output to the detector was peaked.

Flow rates of gas and oxygen to burner 34 were then adjusted to 0.65slpm and 0.6 slpm, respectively, to produce a broader flame having atemperature of about 1650° C. Twelve seconds after the flame wasignited, the oxygen flow was turned off, and stages 45 and 46 pulled inopposite directions at a combined rate of 0.5 cm/sec to further increasethe length of coupler preform 31 by about 0.02 cm. During this step, thelight emanating from fibers 17 and 18 was monitored at 1310 nm. Thestretching operation automatically stopped when the ratio of the opticalpower from fiber 17 to that of fiber 18 was 1.2, at which time thecontrol system instructs the stages to stop moving. Because of systemmomentum, a sufficient amount of stretching continues to occur toprovide a power ratio of 1, whereby equal light power emanated fromfibers 17 and 18 at 1310 nm. The diameter of the midregion is reduced bythe stretching operations as illustrated by region 51 of FIG. 18.

After the coupler had cooled, the vacuum lines were removed from theresultant coupler, and a drops 48 and 49 of heat curable adhesive wereapplied from a syringe to ends 14 and 15, respectively, of the capillarytube. After the adhesive was cured by exposure to heat (arrow H), thecoupler was removed from the draw.

The resultant devices couple approximately 50% of the signal propagatingin that end of optical fiber 17 at end 14 to optical fiber 18 at about1310 nm and 1490 nm; the power slope at 1310 nm is 0.077% per nm or0.006 dB per nm. These couplers exhibited a median excess device loss ofabout 0.3 dB. The lowest measured excess loss was 0.05 dB.

The spectral insertion loss curves for a specific coupler made inaccordance with the specific example are shown in FIG. 19. Curve P₂represents the coupled power. The excess loss for that coupler was 0.09dB and 0.05 dB at 1310 nm and 1550 nm, respectively. The insertion losswas less than 4 dB in each leg of that coupler over a 300 nm range ofwavelengths up to about 1565 nm.

What is claimed is:
 1. An achromatic fiber optic coupler comprisinganelongated body of matrix glass having a refractive index n₃, said bodyhaving two opposed ends and a midregion, a plurality of optical fibersextending longitudinally through said body, each of said fiberscomprising a core of refractive index n₁ and a cladding of refractiveindex less than n₁ but greater than n₃, the refractive index n₂ of thecladding of a first of said fibers being different from the refractiveindex n₂ ' of the cladding of a second of said fibers by such an amountthat the value of Δ_(clads) is greater than zero but less than 0.03%,wherein Δ_(clads) equals (n₂ ^(2-n) ₂ '²)/2n₂ ², said fibers being fusedtogether along with the midregion of said matrix glass, the diameter ofthe central portion of said midregion and the diameters of said opticalfibers in said central portion being smaller than the diameters thereofat the ends of said body, whereby a portion of an optical powerpropagating in one of said fibers couples to the other of said fibers.2. A coupler in accordance with claim 1 wherein the difference betweensaid refractive indices n₂ and n₂ ' is such that the insertion loss isless than 4 dB in each leg thereof over a 300 nm wavelength range
 3. Acoupler in accordance with claim 2 wherein the value of Δ_(clads) isgreater than 0.005%.
 4. A coupler in accordance with claim 2 whereinsaid coupler is capable of coupling about 50% of the power from one ofsaid fibers to the other of said fibers at a predetermined wavelength,and the value of Δ_(clads) is between 0.005% and 0.02%.
 5. A coupler inaccordance with claim 4 wherein Δ₂₋₃ is greater than 0.4%, wherein Δ₂₋₃is equal to (n₃ ² -n₂ ²)/2n₃ ².
 6. A coupler in accordance with claim 2wherein Δ₂₋₃ is greater than 0.4%, wherein Δ₂₋₃ is equal to (n₃ ² -n₂²)/2n₃ ².
 7. A coupler in accordance with claim 1 wherein the differencebetween said refractive indices n₂ and n₂ ' is such that the insertionloss is less than 4 dB in each leg thereof over a 300 nm wavelengthrange.
 8. A coupler in accordance with claim 1 wherein the value ofΔ_(clads) is greater than 0.005%.
 9. A coupler in accordance with claim8 wherein Δ₂₋₃ is greater than 0.4%, wherein Δ₂₋₃ is equal to (n₃ ² -n₂²)/2n₃ ².
 10. A coupler in accordance with claim 1 wherein said coupleris capable of coupling about 50% of the power from one of said fibers tothe other of said fibers at a predetermined wavelength, and the value ofΔ_(clads) is between 0.005% and 0.02%.
 11. A coupler in accordance withclaim 10 wherein Δ₂₋₃ is greater than 0.4%, wherein Δ₂₋₃ is equal to (n₃² -n₂ ²)/2n₃ ².
 12. A coupler in accordance with claim 1 wherein Δ₂₋₃ isbetween 0.4% and 0.65%, wherein Δ₂₋₃ is equal to (n₃ ² -n₂ ²)/2n₃ ². 13.An achromatic fiber optic coupler comprising first and secondsingle-mode glass optical fibers, each having a core and a cladding, therefractive indices n₂ and n₂ ' of said fibers being lower than therefractive index n₁ of said cores, said fibers being fused togetheralong a portion of the lengths thereof to form a coupling region, saidcoupling region being surrounded by matrix glass having a refractiveindex n₃ that is lower than the refractive indices of said claddings,the diameters of said fibers being smaller in said coupling region thanin the remainder of said fibers, and said cores being more closelyspaced in said coupling region than in the remainder of said fibers,thereby forming a coupling region wherein a portion of a signalpropagating in one of said fibers is coupled to the other of saidfibers, the difference between said refractive indices n₂ and n₂ ' beingsuch that the insertion loss is less than 4 dB in each leg thereof overa 300 nm wavelength range.
 14. A coupler in accordance with claim 13wherein the difference between said refractive indices n₂ and n₂ ' issuch that the insertion loss is less than 4 dB in each leg thereofbetween 1265 nm and 1565 nm.
 15. A coupler in accordance with claim 13wherein Δ₂₋₃ is greater than 0.4%, wherein Δ₂₋₃ is equal to (n₃ ² -n₂²)/2n₃ ².
 16. A method of making an achromatic fiber optic couplercomprising the steps ofinserting into a glass tube of refractive indexn₃ at least a portion of each of a plurality of optical fibers so thatsaid portions occupy the midregion of said tube, each of said fiberscomprising a core of refractive index n₁ and a cladding of refractiveindex less than n₁ but greater than n₃, the refractive index n₂ of thecladding of a first of said fibers being different from the refractiveindex n₂ ' of the cladding of a second of said fibers, the differencebetween n₂ and n₂ ' being such that the value of Δ_(clads) is greaterthan zero but less than 0.03%, wherein Δ_(clads) equals

    (n.sub.2.sup.2 -n.sub.2 40 .sup.2)/2n.sub.2.sup.2,

collapsing the midregion of said tube onto fibers, and stretching thecentral portion of said midregion until a predetermined coupling occursbetween said fibers.
 17. A method in accordance with claim 16 whereinthe step of stretching comprises providing relative movement between theends of said tube, and varying the rate at which said relative movementoccurs.
 18. A method in accordance with claim 17 wherein the step ofstretching comprises continuously varying the rate at which saidrelative movement occurs.
 19. A method in accordance with claim 17wherein the step of stretching comprises stretching at one stretch ratefor a first period of time and stretching at another stretch rate for asecond period of time.
 20. A method in accordance with claim 16 whereinthe step of stretching comprises pulling the ends of said tube away fromeach other at a first stretch rate, and before said predeterminedcoupling is achieved, pulling the ends of said tube away from each otherat a second stretch rate that is different from said first stretch rate.21. A method in accordance with claim 16 wherein the step of stretchingcomprises pulling the ends of said tube away from each other at a firststretch rate, and before said predetermined coupling is achieved,pulling the ends of said tube away from each other at a second stretchrate that is less than said first stretch rate.
 22. A method inaccordance with claim 16 wherein the step of stretching comprisesheating the central portion of said tube midregion, stretching thecentral portion of said tube midregion, stopping said stretchingoperation before said predetermined coupling is achieved, reheating thecentral portion of said tube midregion and further stretching thecentral portion of said tube midregion.
 23. A method in accordance withclaim 16 wherein the step of stretching comprises heating the centralportion of said tube midregion to a first temperature, stretching thecentral portion of said tube midregion, stopping said stretchingoperation before said predetermined coupling is achieved, heating thecentral portion of said tube midregion to a second temperature lowerthan said first temperature and stretching the central portion of saidtube midregion.
 24. A method in accordance with claim 16 wherein thestep of stretching comprises heating the central portion of said tubemidregion to a first temperature, stretching the central portion of saidtube midregion at a first stretch rate, stopping said stretchingoperation before said predetermined coupling is achieved, heating thecentral portion of said tube midregion to a second temperature lowerthan said first temperature and stretching the central portion of saidtube midregion at a second stretch rate lower than said first stretchrate.
 25. A method in accordance with claim 16 wherein the step ofstretching comprises stretching the central portion of said tubemidregion at a first stretch rate until some coupling between saidfibers begins to occur, and before said predetermined coupling isachieved, stretching the central portion of said tube midregion at asecond stretch rate different from said first stretch rate.
 26. A methodin accordance with claim 16 wherein said tube has first and second ends,at least a first of said fibers extends from both ends of said tube, andat least a second of said fibers extends from only the second end ofsaid tube, and wherein the step of stretching comprises stretching thecentral portion of said tube midregion until some coupling between saidfibers begins to occur, and using the ratio of the optical power fromsaid fibers to stop said stretching operation.